1. Field of the Invention
Embodiments of the invention generally relate to seismic base isolation systems for buildings and other structures. More specifically, a foundation system that is particularly advantageous to protect lightweight low rise buildings including houses against earthquakes, including those located in low lying wet regions subject to liquefaction.
2. Description of the Related Art
Generally, earthquakes occur all around the world with varying degrees of intensity, sometimes causing severe damage to property and life. Earthquakes in Japan, USA, South America, China and Europe have resulted in massive destruction of human lives and damages to buildings including houses.
Typically, earthquakes cause damage as a result of shaking and vibrations of the earth due to seismic waves generated by a collision of tectonic plates. The geology of the area through which the wave energy passes generally determines the extent of ground shaking Typically, a rocky ground mass may vibrate quickly with small displacements but may resist breakage. Generally, a low lying wet soil may, on the other hand, behave quite differently by shaking enough to allow water particles in soil to build substantial pressure such that soil particles lose cohesion and begin to behave like liquid themselves. This is called liquefaction and is generally responsible for much earthquake damage in certain low-lying wet areas through subsidence, lateral spread of the land, and silt forced to the surface. Typically, layers of sand liquefying below the ground surface under extreme pressure due to overlying sediments may exploit any fissure in the earth's mass under the foundation and burst out on the ground as an eruption of sand. This phenomenon generally known as ‘sand volcano’ may be as devastating for low rise buildings when the sand volcano forces wet sand up through cracks in the foundation or flooring.
Generally, seismic isolation, also known as base isolation, has emerged now days as an important concept and technique of designing buildings that are safe from seismic hazards. Using these concepts, base isolation devices are typically installed between the superstructure above the ground and the foundation which absorb earthquake energy and thereby impede the vibration and displacement of the building. As per a recent review disclosed in Patil, S. J. and G. R. Reddy (2012); State of Art Review—Base Isolation Systems for Structures, International Journal of Emerging Technology and Advanced Engineering (Vol 2, Issue 7, July 2012), base isolation concept was generally coined by engineers and scientists as early as in the year 1923. Since then methods of isolating the buildings and structures from earthquake forces have been developed all over the world. Countries such as the United States, New Zealand, Japan, China and Italy have generally adopted these techniques among the best practices for many public and multi-story residential buildings. Typically, hundreds of buildings are being built every year with base isolation technique in these countries. However due to factors such as weight and cost, typically, base isolation of light weight structures such as one and two story houses is very rare.
Generally, devices used for seismic isolation in buildings are isolators and dampers both of which are now of several different types and are being researched further using different materials and geometries. Typically, there are three main types of isolators:
Elastomeric Isolators that including packaged rubber and metal sheets alternately;
Sliding Isolators, typically made of Teflon™ coated steel sheet; and
Rotating Ball Bearing Isolator that include ball bearing with retainers.
Typically, all of these types of isolators allow moving the building laterally without minimal friction during an earthquake shaking Generally, among the first type of isolators, Lead-Rubber Bearings (LRBs), as disclosed in Skinner, R. I., Robinson, W. H., McVerry, G. H. An Introduction to Seismic Isolation; John Wiley and Sons Inc.: New York, N.Y., USA, 1993, were first introduced and used in New Zealand in the late 1970s. Since then, generally, LRBs have been widely used for seismic isolation around the world including the United States and Japan. Because of cost and technical complexity, typically, seismic isolators are very rarely used in low rise buildings and have been largely limited to large heavy important buildings such as hospitals, office buildings, and museums.
For example, patent and non-patent literature describe a number of base isolation devices with many improvements and associated techniques to handle varying conditions during an earthquake by minimizing the movement of the structure in relation to ground movement.
Despite worldwide popularity and use of known seismic isolation techniques, typically, there are situations when base isolation devices are of little use and fail to provide desired prevention of earthquake damage to buildings. A research study, as disclosed in Thurston, S J (2006); Base Isolation of Low Rise Light and Medium-weight Buildings, BRANZ Study Report No 156 (www.branz.co.nz), examined the question of base isolation of low rise light and medium-weight buildings in New Zealand using commercially available base isolation devices, such as Robal1™ and Roglider™, which are an LBR and a sliding isolator respectively. Generally, the study concluded that the potential use of base isolation systems for New Zealand houses and other low rise buildings is limited since lateral load resistance of the isolation devices is likely to be generally quite low which would prompt significant reduction of design level seismic force in the superstructure, as a result of which most buildings will move excessively under design level or extreme wind forces. Typically, seismic isolation succeeding in such cases would be possible by counter measure of increasing the weight of the structure beyond what is normally required. Thus, generally, conventional seismic isolation may be expensive to implement and maintain in such cases that could be justified if prevention of damage to contents is, prima facie, the objective.
Furthermore, generally, due to the failure of known seismic isolation techniques in situations described above, the occurrence of liquefaction in certain low lying wet soils compound the problem of seismic safety of buildings even further. Typically, liquefaction-induced damages have been observed in many moderate to severe earthquakes, most recently in 2011 in Japan and New Zealand, and earlier in California, Greece, Italy and Turkey.
Generally, according to a key seismic isolation principle, the seismic isolator used should enable the structure above the foundation to have a swing period longer than the vibration period of seismic origin. The phenomenon of liquefaction, on the other hand, typically affects the soil under the foundation and supporting piles. Liquefaction could be prevented, generally, if the pore pressure of soil moisture can be prevented to build up appreciably during seismic vibration such that soil under the foundation continues to remain unsaturated and maintains its frictional resistance as before.
Generally, prior art patent literature describe several techniques to solve the problem of liquefaction in different situations. For example, Japanese Patent 7158044 describes the use of gravel drainage column encased in a synthetic bag pushed through in the ground near the foundation inside a casing pipe which is press-rotated with a mechanical drive and extracted out afterwards. The method is suggested for prevention of liquefaction in the event of an earthquake.
Japanese Patent Publication 2013108253, to Tezuka et al., entitled “A Liquefaction measure Device” describes a more sophisticated liquefaction countermeasure than the above by providing an expendable bladder inside the piers to take in water during a liquefaction phase and keeping the soil around the piers unsaturated. The system of Tezuka et al. also provides earthquake detection means and initiating the expansion of bladder automatically upon detection of earthquake. Other approaches include U.S. Pat. No. 6,308,135, to Hocking, entitled “Soil Liquefaction Prevention by Electro-Osmosis During an Earthquake Event”, describes the use of electro-osmosis to move the water away from foundation, and U.S. Pat. No. 6,659,691, to Berry, entitled “Pile Array Assembly System for Reduced Soil Liquefaction”, discloses a pile array assembly that deflects seismic shock waves thereby densifying the ground to reduce liquefaction.
Generally, the use of piles as support for foundation is not new. For example, prior art techniques describe many instances where piles driven into the ground have been used to support a foundation above the ground. For example, New Zealand Patent 272981, to Melville-Smith, entitled “Improvements in or Relating to Foundations” describes a methodology of constructing a concrete foundation on the surface of the ground resting on piles, which are preferably wooden and driven into the ground; wherein such a foundation also acting as floor upon which building structure can be erected. The improvement appears to use piles to replace the cement footing in excavated ground. The invention, however, appears to lack any mention of seismic activity or resilience and the cement slab foundations on the ground surface would be prone to movement and cracking during an earthquake.
Generally, houses built on timber or concrete piles are quite common in New Zealand and elsewhere in the world. For example, the current New Zealand Standard for Timber Framed Buildings, NZS3604:2011, applicable to all new construction prescribes, among other things, specific requirements for ground type and bearing capacity, pile layout, pile height above ground, pile type & size etc. In the aftermath of recent Christchurch earthquake and widespread damage of building properties, the awareness campaign launched by the Government of New Zealand through a web-based guidance note entitled, ‘Earthquake Strengthen Your House’, generally advises on periodic check of timber and concrete piles that may have been damaged and/or dislocated away from the bearers. It is also recommended to ensure that piles are directly and properly secured to the bearers through z-nails or braces.
Liquefaction associated failure may be of the following types:
Tilting due to instability,
Direct settlement due to loss of bearing capacity,
Uplift due to buoyancy effects and
Structural deformation due to lateral spread of the ground.
Generally, these phenomena are now better understood as a result of detailed studies on the causes and impact of liquefaction on major infrastructure projects like bridges etc. after major earthquakes in recent years in different parts of the world. A number of mitigation and countermeasures to the effect of liquefaction such as compaction techniques now recommended commonly for large projects are, however, generally not practicable and cost-effective for small residential housing projects.
Typically, buildings with foundation of concrete platform resting on piles or excavated footings have shown that current methods are not effective in certain areas where the building rests on piles or footings in a liquefiable soil and are not capable to withstand the effect of a moderate to strong earthquake without extensive and expensive ground preparation. Generally, when the bottom of the foundation platform is cracked, liquefaction causes water and mud to erupt upwards with force through cracks and above the foundation, making the building no longer habitable. If liquefaction has caused the slab to subside, then typically very expensive re-leveling will be required and repairs to the structure due to foundation subsidence can exceed the cost of building a new equivalent house.
Generally, foundations using piles and attached floor joists or bearers are subject to racking and deforming during an earthquake causing shifts in floor levels and related damage to the structure of the buildings. The racking and deforming, typically, may be increased in liquefiable soils due to greater horizontal and vertical movement of the piles.
Therefore, in view of the above, there is a need for an alternate solution for foundation for light weight structures and buildings in low lying wet regions such as those of north-eastern region of India in Assam or lands of technical category 3 of Canterbury region in New Zealand that have been officially assessed to experience moderate to significant land damage from liquefaction in future large earthquakes, or at least provide the public with a choice.
Further aspects and advantages of the system will become apparent from the ensuing description that is given by way of example only.